Add trending links (#16917)

* Add trending links

* Add overriding specific links trendability

* Add link type to preview cards and only trend articles

Change trends review notifications from being sent every 5 minutes to being sent every 2 hours

Change threshold from 5 unique accounts to 15 unique accounts

* Fix tests

app/models/trends.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+module Trends
+  def self.table_name_prefix
+    'trends_'
+  end
+
+  def self.links
+    @links ||= Trends::Links.new
+  end
+
+  def self.tags
+    @tags ||= Trends::Tags.new
+  end
+
+  def self.refresh!
+    [links, tags].each(&:refresh)
+  end
+
+  def self.request_review!
+    [links, tags].each(&:request_review) if enabled?
+  end
+
+  def self.enabled?
+    Setting.trends
+  end
+end